Cholesterol metabolism Structure of cholesterol OH is polar part The ring is non polar part Cholesterol ester (CE) is completely non polar

Functions of cholesterol: 1- Enter in the structure of every cell (cell membrane). 2- Gives vitamin D 3 by UV radiation 3- Gives bile acids and salts in liver and this is activated by thyroid hormone. 4- Give steroidal hormones (hormones of adrenal cortex and sex hormones).

Sources of cholesterol in our body: Exogenous or Dietary source: it is present in egg yolk, meat, burger, liver and brain. Endogenous or from synthetic pathway in liver (mainly), intestine, adrenal cortex, ovaries, testis and skin.

Digestion occurs mainly in intestine Most dietary cholesterol is present in free form (not estrified) with 10-15% present as cholesterol ester (CE, fatty acid attached to OH at C3). Since free cholesterol is more absorbable (can penetrate the water layer surrounding the enterocytes ), so all CE should be converted into free cholesterol Cholesterol esterase secreted in pancreatic juice acts on cholesterol ester giving free cholesterol and fatty acids. Bile salts are necessary for activation of this enzyme. CE free Cholesterol + FA I- Digestion and absorption of dietary cholesterol esterase

In intestinal mucosa, the major amounts of free cholesterol (about 80-90%) combine with acyl CoA (active form of fatty acids) to form cholesterol esters which transported in blood with TAG, phospholipids and apoprotein to form lipoprotein chylomicron. The enzyme that catalyzes cholesterol esterification in mucosa is ACAT (Acyl CoA: Cholesterol Acyl Transferase).

II- Synthesis of cholesterol: De novo synthesis of cholesterol occurs in all body cells but mainly in the cytoplasm and ER of liver and intestine. Synthesis also occur in adrenal cortex, ovary, testis, placenta, skin. The precursor is acetyl CoA derived from oxidation of glucose in mitochondrial matrix (as occur in FA synthesis). SO in fed state (high CHO diet) the excess acetyl CoA produced from glucose oxidation is used for biosynthesis of both FA and cholesterol - Acetyl CoA must be transported from mitochondria to cytoplasm as citrate (as in fatty acid synthesis).then citrate is cleaved in the cytoplasm by citrate cleavage system (exactly as occur in FA synthesis)

Transport of acetyl CoA from mitochondria to cytoplasm: Mitochondria: OAA + Acetyl CoA -CoA ↓ citrate synthase Citrate Inner mitochondrial membrane ↓ Citrate + CoA, ATP ↓ ATP citrate lyase OAA + Acetyl CoA Cystosol

The process of cholesterol synthesis has these major steps: 1.Two molecules of acetyl CoA condense together to form acetoacetyl CoA. 2.Acetoacetyl CoA condense with third molecule of acetyl CoA to form hydroxymethyl glutaryl CoA ( HMG-CoA), the enzyme is cytosolic HMG CoA synthase 3. HMG-CoA is reduced to mevalonate (6C), the enzyme is HMG CoA reductase which is the key enzyme in the biosynthesis 4. Mevalonate is phosphorylated and decarboxylated to form the isoprene unit [or called isopentenyl pyroposphate] (5C).

4. Two isoprene units condense to form geranyl pyrophosphate (10 C) which react with another 5C unit to from Farnesyl pyrophosphate (15C) 5. Two molecules of farnensyl condense to form squalene (30 C) which cyclise to form Lanosterol 6. Lanosterol is converted into cholesterol by series of reactions.

Mevalonate (6C) ↓ phosphprylation and decaboxylation Isoprene unit (5C) [or called isopentenyl pyroposphate] ↓ Geranyl PP (10 C) ↓ Feransyl PP (15 C) ↓ Squalene (30 C) ↓cyclization Lanosterol (30 C) ↓ - 3CO 2 Cholesterol (27 C)

cytoplasmic H MG-CoA synthase (different from that responsible for ketogenesis) Cholesterol synthesis

Regulation of cholesterol synthesis: HMG- CoA reductase is the key (rate limiting) enzyme in the biosynthesis of cholesterol. It is regulated by: 1- Feed back inhibition by cholesterol: Cholesterol (the end product of the pathway) acts as feed back inhibitor of the pre-existing HMG –CoA reductase as well as inducing rapid degradation of the enzyme.. 2- Drug inhibition: Statins such as atorvastatin (by Pfizer), lovastatin and simvastatin are drugs with a side chain structurally similar to HMG-CoA so competitively inhibit HMG-CoA reductase. They are used to decrease cholesterol levels in patients with hypercholesterolemia. 3- Diet: its activity activated by high CHO and fat diet and inhibited in starvation. 4- Hormonal regulation: insulin stimulate protein phosphatase enzyme which phosphorylates the enzyme and actives it (dephosphprylated form is the active form) antiinsulin hormons such as glucagon inactivate the enzyme. (it similar to regulation of acetyl Co A carboxylase enzyme in FA synthesi)

Degradation of cholesterol Ring of sterol can’t be metabolized to CO 2 & H 2 O in humans. The excess cholesterol is eliminated from the body by conversion to bile acids and bile salts (80-90%) or by secretion into bile (10-20%) then to intestine for elimination. Conversion of cholesterol into bile acids and bile salts: 80-90% of cholesterol is converted (oxidized) into bile acids then to bile salts. This is activated by thyroxin hormone. So hypothyrodism leads to increased cholesterol in blood.

Bile acids: Two bile acids are synthesized in liver from cholesterol by the action 7-α hydroxylase enzyme and then further hydorxylation and oxidation of side chain, the produced bile acids are: cholic acid and chenodeoxy cholic acid. chenodeoxycholic acid and cholic acid are called primary bile acids.

OH Synthesis of primary bile acids in the liver (for your knowledge) OH

Formation of bile salts: Cholic acid and chenodeoxycholic acid are conjugated in liver with either glycine or taurine (formed from cysteine amino acid) forming glycocholic acid or taurocholic acid. Bile salts are carried from the liver to gallbladder, where they are stored for future use. Bile salts have more effective emulsifying effect thean bile acids.

Enterohepatic Circulation of Bile Acids and Salts Cholic acid, chenodeoxycholic acid and their conjugates in liver are secreted into intestine to help lipid digestion. In intestine, some bile salts are deconjugated to get bile acids. Some primary bile acids are dehydroxylated by intestinal bacteria to the secondary bile acids, identified as deoxycholic acid and lithocholic acid. Entrohepatic circulation the bile salts 95 % of primary and secondary bile acids and salts are returned to the liver, where they can be reconjugated with glycine or taurine. 5% of bile acids are lost in the feces

Functions of bile salts in fat metabolism: 1. their synthesis and subsequent excretion in the feces represent the only significant mechanism for the elimination of excess cholesterol. 2. Bile salts and phospholipids solubilize cholesterol in the bile, thereby preventing the precipitation of cholesterol in the gallbladder. So deficiency of bile salts leads to cholesterol gallstones. 3. they facilitate the digestion of dietary triacylglycerols by acting as emulsifying agents that render fats accessible to pancreatic lipases. 4. Activate pancreatic lipase (steapsin) and cholesterol esterase

Disturbances in cholesterol metabolism: Plasma cholesterol: normal cholesterol level in blood is mg/dL (average 200 mg/dL). Plasma cholesterol is derived mainly from liver and intestine. Hypercholesterolemia: i.e increased cholesterol level in blood than 250 mg/dL: Causes: 1- Hypothyrodism: lead to decreased conversion of cholesterol into bile acids and decreased mobilization of cholesterol from blood to tissues. 2- Diabetes: due to increased absorption of dietary cholesterol 3- Diet rich in carbohydrates, and fats: increase the synthesis of cholesterol in liver due to: a - Increased the activity of the key enzyme (HMG-CoA reductase). b - Formation of excess acetyl CoA (from fats and CHO) than the requirements of kreb’s cycle. c- insulin secreted after CHO diet will activate the key enzyme (dephosphorylate it) 4- High cholesterol in diet

Hypocholesterolemia: decrease of cholesterol level in blood below 150 mg/dL: 1- Hyperthyrodism: ↑ thyroid hormone→ a- ↑ oxidation of cholesterol into bile salts. b- increase mobilization of cholesterol from blood to tissues. 2-Starvation: inhibits cholesterol synthesis by decreasing HMG- CoA reductase. 3- Estrogens: decrease cholesterol and prevent atherosclerosis, so, coronary heart disease rarely occurs in females during reproductive period of life.

Study question: High carbohydrate diet may lead to all of the following EXCEPT: a) release of insulin which inactivate HMG CoA reductase b) Hypercholesterolemia c) Over synthesis of fatty acids and TAG d) Increased synthesis of acetyl CoA carboxylase